HEARTTHROB HARMON SUCCEEDS ON HIS OWN

Actor Mark Harmon had a sinecure on television's St. Elsewhere series, working three days a week for a hefty salary with job security and the status of national dreamboat.

Handsome and athletic, the 34-year-old bachelor had it made.

But then, two years ago, the son of football immortal Tom Harmon and actress Elyse Knox chucked it all for the insecure business of movies.

"It was a tough decision," Harmon said the other day. "I'd been on the series three years and it felt pretty comfortable. It's never easy for an actor to give up job security and a paycheck.

"But this is not a secure business. I'm not an actor because I'm seeking security."

Although he was comfortable in the role of young Dr. Robert Caldwell, Harmon, now 36, was dissatisfied with his contribution to the show. In his final year with the series he asked the producers and writers to challenge him as an actor.

He suggested they radically change the character and personality of Caldwell at a time when other cast members were content simply to seek higher salaries.

"I questioned working three days a week in a large ensemble cast instead of putting in five days a week," he said. "They warned me that if they wrote Caldwell to the max there would be no place to go except out of the show.

"I said OK, and they changed the character 180 degrees. And they did write me out when Caldwell died of AIDS.

"It seemed to me it would be good for the ratings to put that dramatic an ending to the character, but I didn't know if it would be good for me. I had no work waiting in the wings.

"As I sit here now, I realize it was the right move, although I had no way of knowing at the time."

Before leaving St. Elsewhere, Harmon had co-starred with Robert Duvall and Gary Busey in Let's Get Harry, a movie that never made it to theaters.

"That picture was disappointing and frustrating," he said. "You put just as much effort into a bad film as a good one, and the line between the two is very thin."

His first project after the series was the four-hour NBC miniseries Deliberate Stranger, in which he played serial killer Theodore Bundy, very much against his good-guy image.

He returned to television again for three segments of Moonlighting, playing a strong romantic character, and for the ABC-TV movie The Prince of Bel Air, which did well in the ratings.

Harmon now can be seen in Summer School, a light comedy that also marks the first time he gets top billing. While Summer School will not win any Oscars, it is aimed at the youth audience without sex, nudity, drugs or food fights.

"I took the part because I wanted to work for a director like Carl Reiner," Harmon said. "I'm in almost every scene and it gave me an opportunity to play a different kind of guy."

He portrays an easygoing gym teacher and surfer at a seaside high school who gets stuck teaching English to a bunch of knuckleheads who flunked the course during the regular semester.

"I told Carl at the beginning I wasn't a broad-based, stand-up comedian," Harmon said. "He told me not to worry, that he wanted a leading man who knew where the jokes were.

"I chose Summer School because actors should make choices based on a package that can give them some help. In addition to Carl, I liked the script and the fact that Paramount would be releasing the picture."

Asked about his sex symbol status, Harmon grinned. He said he still receives hundreds of letters from admiring female fans, and women still approach him in person since his marriage four months ago to actress Pam Dawber.

"Things haven't changed," he said, "and a guy has to enjoy it."

Harmon will be seen next in the CBS movie After The Promise before starting work with Jodie Foster in the Warner Bros. family drama Stealing Home. In January, he will star in Bus Stop on stage at the Los Angeles Music Center.

"I am as busy as I want to be," he said. "I had no idea I would have so many projects going when I left St. Elsewhere. But I thought things might work out if I took some chances -- and that's what's happening."